The Insulin-Sensitive Glucose Transporter

Reprogramming of glucose metabolism in virus infected cells

Viral infection is a kind of cellular stress that leads to the changes in cellular metabolism. Many metabolic pathways in a host cell such as glycolysis, amino acid and nucleotide synthesis are altered following virus infection. Both oncogenic and non-oncogenic viruses depend on host cell glycolysis for their survival and pathogenesis. Recent studies have shown that the rate of glycolysis plays an important role in oncolysis as well by oncolytic therapeutic viruses. During infection, viral proteins interact with various cellular glycolytic enzymes, and this interaction enhances the catalytic framework of the enzymes subsequently the glycolytic rate of the cell. Increased activity of glycolytic enzymes following their interaction with viral proteins is vital for replication and to counteract the inhibition of glycolysis caused by immune response. In this review, the importance of host cell glycolysis and the modulation of glycolysis by various viruses such as oncogenic, non-oncogenic and oncolytic viruses are presented.

Scopoletin increases glucose uptake through activation of PI3K and AMPK signaling pathway and improves insulin sensitivity in 3T3-L1 cells

Coumarins have been shown to reduce blood glucose levels and improve insulin sensitivity in other studies. The purpose of this study was to investigate the effects of scopoletin, which is a type of coumarin family, on glucose uptake in 3T3-L1 cells to test the hypothesis that scopoletin exerts an antidiabetic function on adipocytes. Scopoletin significantly increased glucose uptake, which was associated with increased expression of the plasma membrane glucose transporter type 4 (PM-GLUT4) in 3T3-L1 adipocytes. This increase in PM-GLUT4 expression was promoted by phosphorylation of protein kinase B, activation of phosphatidylinositol-3-kinase (PI3K), and enhanced intracellular glucose uptake. Scopoletin also promoted phosphorylation of adenosine monophosphate-activated protein kinase (AMPK) and enhanced PM-GLUT4 expression. Scopoletin-induced glucose uptake in 3T3-L1 adipocytes was inhibited by treatment with the PI3K inhibitor wortmannin and the AMPK inhibitor compound C. These results suggest that scopoletin has an antidiabetic effect by stimulating GLUT4 translocation to the PM through activation of the PI3K and AMPK pathways in 3T3-L1 adipocytes, thereby upregulating glucose uptake.

Pharmacodynamic characterization of insulin on MDMA-induced thermogenesis

Sympathomimetic drugs (MDMA; ecstasy) induce a potentially catastrophic hyperthermia that involves free fatty acid (FFA) activation of mitochondrial uncoupling proteins (UCP). Insulin is an important regulator of plasma FFA levels, although its role in thermogenesis is unclear. The aims of the present study were 1) to characterize the pharmacodynamic effects of MDMA on plasma insulin and glucose, 2) to examine the effects of insulin on MDMA-induced thermogenesis and 3) to examine MDMA-induced thermogenesis in an animal model of insulin resistance, the obese Zucker rat. Insulin levels peaked 15 min after MDMA (40 mg/kg, s.c.), which preceded the peak temperature change at 60 min. Plasma glucose levels also peaked 15 min. after MDMA and remained elevated throughout the 90-min. monitoring period. Insulin pretreatment (10 units/kg, s.c.) 30 min. before a low dose of MDMA (5 mg/kg, s.c.) potentiated the thermogenic response. Insulin resistant, fa/fa (obese) Zucker rats demonstrated an attenuated thermogenic response to MDMA (40 mg/kg, s.c.). Consistent with the role for FFA in UCP3 expression, immunoblot analysis showed significantly increased levels of UCP3 protein obese compared to lean Zucker skeletal muscle. In conclusion, the results of the present study suggest a potential role of insulin signaling in sympathomimetic-induced thermogenesis.

Application of immunocytochemistry and immunofluorescence techniques to adipose tissue and cell cultures

When isolated from tissue, white adipose cells are round, and their interior is filled with a large (80-120 microm) droplet of stored triglyceride, leaving a thin (1-2-microm) layer of cytoplasm between the lipid droplet and the plasma membrane. Their three-dimensional architecture, together with the fact that these cells ordinarily float in medium, have created major challenges when one attempts to perform microscopy techniques with these cells. Adipocytes serve as the principal energy reservoir in the body, and it is essential to overcome these difficulties to be able to study hormone-mediated responses in real adipose cells, which convey physiological significance that cannot be readily duplicated by the use of cultured model adipocytes. This chapter focuses on the use of confocal microscopy optical sectioning and computer-assisted image reconstruction in the whole adipose cell in the study of insulin-regulated protein trafficking. In addition, we illustrate the possibility to image whole-mount preparations of living adipose tissue, opening new ways to probe adipose cells in situ without disrupting their cellular interactions within living adipose tissue. Confocal microscopy constitutes an effective morphological approach to investigating adipose cell physiology and pathophysiology.

Cell signaling, the essential role of O-GlcNAc!

An increasing body of evidence points to a central regulatory role for glucose in mediating cellular processes and expands the role of glucose well beyond its traditional role(s) in energy metabolism. Recently, it has been recognized that one downstream effector produced from glucose is UDP-GlcNAc. Levels of UDP-GlcNAc, and the subsequent addition of O-linked beta-N-acetylglucosamine (O-GlcNAc) to Ser/Thr residues, is involved in regulating nuclear and cytoplasmic proteins in a manner analogous to protein phosphorylation. O-GlcNAc protein modification is essential for life in mammalian cells, highlighting the importance of this simple post-translational modification in basic cellular regulation. Recent research has highlighted key roles for O-GlcNAc serving as a nutrient sensor in regulating insulin signaling, the cell cycle, and calcium handling, as well as the cellular stress response.

Synergistic effect of prostaglandin F2alpha and cyclic AMP on glucose transport in 3T3-L1 adipocytes

The combined effect of prostaglandin F2alpha (PGF2alpha) and cAMP on glucose transport in 3T3-L1 adipocytes was examined. In cells pretreated with PGF2alpha and 8-bromo cAMP for 8 h, a synergy between these two agents on glucose uptake was found. Insulin-stimulated glucose transport, on the other hand, was only slightly affected. The synergistic effect of these two agents was suppressed in the presence of cycloheximide and actinomycin D. In concord, immunoblot and Northern blot analyses revealed that GLUT1 protein and mRNA levels were both increased in cells pretreated with both PGF2alpha and 8-bromo cAMP, greater than the additive effect of each agent alone. The synergistic action of PGF2alpha with 8-bromo cAMP to enhance glucose transport was inhibited by GF109203X, a selective protein kinase C (PKC) inhibitor. In addition, in cells depleted of diacylglycerol-sensitive PKC by prolonged treatment with 4beta-phorbol 12beta-myristate 13alpha-acetate, a PKC activator, the synergistic effects of PGF2alpha and 8-bromo cAMP on glucose transport and GLUT1 mRNA accumulation were both abolished. Taken together, these results indicate that PGF2alpha may act with cAMP in a synergistic way to increase glucose transport, probably through enhanced GLUT1 expression by a PKC-dependent mechanism.

Regulation of insulin-responsive aminopeptidase expression and targeting in the insulin-responsive vesicle compartment of glucose transporter isoform 4-deficient cardiomyocytes

In adipocytes and cardiac or skeletal muscle, glucose transporter isoform 4 (GLUT4) is targeted to insulin-responsive intracellular membrane vesicles (IRVs) that contain several membrane proteins, including insulin-responsive aminopeptidase (IRAP) that completely colocalizes with GLUT4 in basal and insulin-treated cells. Cardiac GLUT4 content is reduced by 65-85% in IRAP knockout mice, suggesting that IRAP may regulate the targeting or degradation of GLUT4. To determine whether GLUT4 is required for maintenance of IRAP content within IRVs, we studied the expression and cellular localization of IRAP and other GLUT4 vesicle-associated proteins, in hearts of mice with cardiac-specific deletion of GLUT4 (G4H-/-). In G4H-/- hearts, IRAP content was reduced by 60%, but the expression of other vesicle-associated proteins, namely cellugyrin, IGF-II/mannose-6-phosphate, and transferrin receptors, secretory carrier-associated membrane proteins and vesicle-associated membrane protein were unchanged. Using sucrose gradient centrifugation and cell surface biotinylation, we found that IRAP content in 50-80S vesicles where GLUT4 vesicles normally sediment was markedly depleted in G4H-/- hearts, and the remaining IRAP was found in the heavy membrane fraction. Although insulin caused a discernible increase in cell surface IRAP content of G4H-/- cardiomyocytes, cell surface IRAP remained 70% lower than insulin-stimulated controls. Immunoabsorption of intracellular vesicles with anticellugyrin antibodies revealed that IRAP content was reduced by 70% in both cellugyrin-positive and cellugyrin-negative vesicles. Endosomal recycling, as measured by transferrin receptor recycling was normal. Thus, GLUT4 and IRAP content of early endosome-derived sorting vesicles and of IRVs are coordinately regulated, and both proteins are required for maintenance of key constituents of these compartments in cardiac muscle cells in vivo.

Prostaglandin F2α increases glucose transport in 3T3-L1 adipocytes through enhanced GLUT1 expression by a protein kinase C-dependent pathway

The effect of prostaglandin F2alpha (PGF2alpha) on glucose transport in differentiated 3T3-L1 adipocytes was examined. Whereas PGF2alpha had little influence on insulin-stimulated 2-deoxyglucose uptake, it increased basal glucose uptake in a time- and dose-dependent manner, reaching maximum at approximately 8 h. The long-term effect of PGF2alpha on glucose transport was inhibited by both cycloheximide and actinomycin D. In concord, while the content of GLUT4 protein was not altered, immunoblot and Northern blot analyses revealed that both GLUT1 protein and mRNA levels were increased by exposure of cells to PGF2alpha. The effect of PGF2alpha on glucose uptake was inhibited by GF109203X, a selective protein kinase C (PKC) inhibitor. In addition, in cells depleted of diacylglycerol-sensitive PKC by prolonged treatment with 4beta-phorbol 12beta-myristate 13alpha-acetate (PMA), the stimulatory effects of PGF2alpha on glucose transport and GLUT1 mRNA accumulation were both inhibited. In accord, PMA was shown to stimulate GLUT1 mRNA accumulation. To further investigate if PKC may be activated by PGF2alpha, we tested several diacylglycerol-sensitive PKC isozymes and found that PGF2alpha was able to activate PKCepsilon. Taken together, these results indicate that PGF2alpha may enhance glucose transport in 3T3-L1 adipocytes by stimulating GLUT1 expression via a PKC-dependent mechanism.

Expression of constitutively activated Akt in the mammary gland leads to excess lipid synthesis during pregnancy and lactation

Expression of constitutively activated Akt in the mammary glands of transgenic mice results in a delay in post-lactational involution. We now report precocious lipid accumulation in the alveolar epithelium of mouse mammary tumor virus-myr-Akt transgenic mice accompanied by a lactation defect that results in a 50% decrease in litter weight over the first 9 days of lactation. Although ductal structures and alveolar units develop normally during pregnancy, cytoplasmic lipid droplets appeared precociously in mammary epithelial cells in early pregnancy and were accompanied by increased expression of adipophilin, which is associated with lipid droplets. By late pregnancy the lipid droplets had become significantly larger than in nontransgenic mice, and they persisted into lactation. The fat content of milk from lactating myr-Akt transgenic mice was 65-70% by volume compared to 25-30% in wild-type mice. The diminished growth of pups nursed by transgenic mothers could result from the high viscosity of the milk and the inability of the pups to remove sufficient quantities of milk by suckling. Transduction of the CIT3 mammary epithelial cell line with a recombinant human adenovirus encoding myr-Akt resulted in an increase in glucose transport and lipid biosynthesis, suggesting that Akt plays an important role in regulation of lipid metabolism.

Cdc42 is a Rho GTPase family member that can mediate insulin signaling to glucose transport in 3T3-L1 adipocytes

We investigated the role of cdc42, a Rho GTPase family member, in insulin-induced glucose transport in 3T3-L1 adipocytes. Microinjection of anti-cdc42 antibody or cdc42 siRNA led to decreased insulin-induced and constitutively active G(q) (CA-G(q); Q209L)-induced GLUT4 translocation. Adenovirus-mediated expression of constitutively active cdc42 (CA-cdc42; V12) stimulated 2-deoxyglucose uptake to 56% of the maximal insulin response, and this was blocked by treatment with the phosphatidylinositol 3-kinase (PI3-kinase) inhibitor, wortmannin, or LY294002. Both insulin and CA-G(q) expression caused an increase in cdc42 activity, showing that cdc42 is activated by insulin and is downstream of G alpha(q/11) in this activation pathway. Immunoprecipitation experiments showed that insulin enhanced a direct association of cdc42 and p85, and both insulin treatment and CA-cdc42 expression stimulated PI3-kinase activity in immunoprecipitates with anti-cdc42 antibody. Furthermore, the effects of insulin, CA-G(q), and CA-cdc42 on GLUT4 translocation or 2-deoxyglucose uptake were inhibited by microinjection of anti-protein kinase C lambda (PKC lambda) antibody or overexpression of a kinase-deficient PKC lambda construct. In summary, activated cdc42 can mediate 1) insulin-stimulated GLUT4 translocation and 2) glucose transport in a PI3-kinase-dependent manner. 3) Insulin treatment and constitutively active G(q) expression can enhance the cdc42 activity state as well as the association of cdc42 with activated PI3-kinase. 4) PKC lambda inhibition blocks CA-cdc42, CA-G(q), and insulin-stimulated GLUT4 translocation. Taken together, these data indicate that cdc42 can mediate insulin signaling to GLUT4 translocation and lies downstream of G alpha(q/11) and upstream of PI3-kinase and PKC lambda in this stimulatory pathway.

Indinavir acutely inhibits insulin-stimulated glucose disposal in humans: a randomized, placebo-controlled study

BACKGROUND

Therapy with HIV protease inhibitors (PI) causes insulin resistance even in the absence of HIV infection, hyperlipidemia or changes in body composition. The mechanism of the effects on insulin action is unknown. In vitro studies suggest that PI selectively and rapidly inhibit the activity of the insulin-responsive glucose transporter GLUT-4. We hypothesized that a single dose of the PI indinavir resulting in therapeutic plasma concentrations would acutely decrease insulin-stimulated glucose disposal in healthy human volunteers.

METHODS

Randomized, double-blind, cross-over study comparing the effect of 1200 mg of orally administered indinavir and placebo on insulin-stimulated glucose disposal during a 180-min euglycemic, hyperinsulinemic clamp. Six healthy HIV-seronegative adult male volunteers were studied twice with 7 to 10 days between studies.

RESULTS

There were no significant differences in baseline fasting body weight, or plasma glucose, insulin, lipid and lipoprotein levels between placebo- and indinavir-treated subjects. During steady-state (t60-180 min) insulin reached comparable levels (394 +/- 13 versus 390 +/- 11 pmol/l) and glucose was clamped at approximately 4.4 mmol/l under both conditions. The average maximum concentration of indinavir was 9.4 +/- 2.2 microM and the 2-h area under the curve was 13.5 +/- 3.1 microM.h. Insulin-stimulated glucose disposal per unit of insulin (M/I) decreased in all subjects from 14.1 +/- 1.2 to 9.2 +/- 0.8 mg/kg.min per microUI/ml (95% confidence interval for change, 3.7-6.1; P < 0.001) on indinavir (average decrease, 34.1 +/- 9.2%). The non-oxidative component of total glucose disposal (storage) decreased from 3.9 +/- 1.8 to 1.9 +/- 0.9 mg/kg.min (P < 0.01). Free fatty acid levels were not significantly different at baseline and were suppressed equally with insulin administration during both studies.

CONCLUSIONS

A single dose of indinavir acutely decreases total and non-oxidative insulin-stimulated glucose disposal during a euglycemic, hyperinsulinemic clamp. Our data are compatible with the hypothesis that an acute effect of indinavir on glucose disposal in humans is mediated by a direct blockade of GLUT-4 transporters.

Isolation and characterization of the two major intracellular Glut4 storage compartments

In rat adipose cells, intracellular Glut4 resides in two distinct vesicular populations one of which contains cellugyrin whereas another lacks this protein (Kupriyanova, T. A., and Kandror, K. V. (2000) J. Biol. Chem. 275, 36263--36268). Cell surface biotinylated MPR and (125)I-labeled transferrin are accumulated in cellugyrin-positive vesicles and to a lesser extent in cellugyrin-negative vesicles. An average cellugyrin-positive vesicle carries not more than one molecule of either Glut4, insulin-responsive aminopeptidase (IRAP), or transferrin receptor (TfR), whereas cellugyrin-negative vesicles contain five to six molecules of Glut4, more than 10 molecules of IRAP, and still one molecule of TfR per vesicle. Cellugyrin-negative vesicles are translocated to the cell surface after insulin stimulation, whereas cellugyrin-positive vesicles maintain intracellular localization both in the absence and in the presence of insulin and, therefore, may be involved in interendosomal protein transport. Both cellugyrin-positive and cellugyrin-negative vesicles are present in extracts of non-homogenized cells and therefore may represent the major form of Glut4 storage in vivo.

Immunocytochemical demonstration of glucose transporters in epiphyseal growth plate chondrocytes of young rats in correlation with autoradiographic distribution of 2-deoxyglucose in chondrocytes of mice

The epiphyseal growth plate, where chondrocytes proliferate and differentiate, is the major site for longitudinal bone growth, matrix synthesis and mineralization. Glucose is an important energy source for the metabolism and growth of chondrocytes. The family of facilitative glucose transporters (GLUTs) mediates glucose transport across the plasma membrane in mammalian cells. We used immunocytochemical methods with anti-GLUT antibodies to investigate the localization of GLUTs in chondrocytes of the epiphyseal growth plate in 3 age groups of rats (3, 7, and 28 days after birth). Intense immunoreactivity of GLUT isoforms 1-5 was detected in chondrocytes of 3-day and 7-day old rats, and all GLUTs were localized in the maturation zone of the hypertrophic zone. On postnatal day 28, chondrocytes in the maturation zone showed intense GLUT1, 4 and 5 immunoreactivity, and weak GLUT2 and 3 immunoreactivity. In addition to chondrocytes in the maturation zone, those in the degenerative zone and in the zone of provisional calcification showed strong GLUT4 and 5 immunoreactivity. Autoradiography of bone sections from 4-week old mice injected with 14C-2-deoxyglucose showed high silver grain density within matrix tissue in the reserve and proliferative zones but not around chondrocytes. However, in the hypertrophic zone, silver grain density was high in matrix and chondrocytes. These data indicate that chondrocytes in the hypertrophic zones use glucose as energy source. High levels of GLUT4 expression imply that glucose use in chondrocytes is regulated by insulin. Expression of GLUT5 in chondrocytes suggests that fructose is also used as an energy source.

A Glut4-vesicle marker protein, insulin-responsive aminopeptidase, is localized in a novel vesicular compartment in PC12 cells

Glut4-containing vesicles represent a regulated recycling compartment in insulin-sensitive fat and skeletal muscle cells, the nature and origin of which are not fully understood. In addition to Glut4 itself, these vesicles compartmentalize a number of proteins, at least one of which, insulin-responsive aminopeptidase, or IRAP, is completely colocalized with Glut4 in insulin-sensitive tissues. However, unlike Glut4, IRAP is expressed in a variety of other tissues and cell lines. Here, we explored the intracellular localization of IRAP in the rat pheochromocytoma cell line PC12. We found that this protein is present in a distinct population of slowly recycling light vesicles. By gradient centrifugations, immunoadsorption and double immunofluorescent staining, these vesicles are different from transferrin-containing endosomes, small synaptic vesicles and secretory granules and may thus represent a novel compartment in PC12 cells. Glut4-GFP chimera transiently expressed in PC12 cells is targeted to IRAP-containing vesicles indicating that cotargeting of Glut4 and IRAP is not specific for adipocytes and myocytes, but is faithful in a foreign cell type. We suggest that PC12 cells may possess a novel type of a vesicular carrier that may represent the homolog of Glut4-vesicles.

ADP-ribosylation factor 6 delineates separate pathways used by endothelin 1 and insulin for stimulating glucose uptake in 3T3-L1 adipocytes

In 3T3-L1 adipocytes, both insulin and endothelin 1 stimulate glucose transport via translocation of the GLUT4 glucose carrier from an intracellular compartment to the cell surface. Yet it remains uncertain as to whether both hormones utilize identical pathways and to what extent each depends on the heterotrimeric G protein Galphaq as an intermediary signaling molecule. In this study, we used a novel inducible system to rapidly and synchronously activate expression of a dominant inhibitory form of ADP-ribosylation factor 6, ARF6(T27N), in 3T3-L1 adipocytes and assessed its effects on insulin- and endothelin-stimulated hexose uptake. Expression of ARF6(T27N) in 3T3-L1 adipocytes was without effect on the ability of insulin to stimulate either 2-deoxyglucose uptake or the translocation of GLUT4 or GLUT1 to the plasma membrane. However, the same ARF6 inhibitory mutant blocked the stimulation of hexose uptake and GLUT4 translocation in response to either endothelin 1 or an activated form of Galphaq, Galphaq(Q209L). These results suggest that endothelin stimulates glucose transport through a pathway that is distinct from that utilized by insulin but is likely to depend on both a heterotrimeric G protein from the Gq family and the small G protein ARF6. These data are consistent with the interpretation that endothelin and insulin stimulate functionally different pools of glucose transporters to be redistributed to the plasma membrane.

Insulin resistance and a diabetes mellitus-like syndrome in mice lacking the protein kinase Akt2 (PKB beta)

Glucose homeostasis depends on insulin responsiveness in target tissues, most importantly, muscle and liver. The critical initial steps in insulin action include phosphorylation of scaffolding proteins and activation of phosphatidylinositol 3-kinase. These early events lead to activation of the serine-threonine protein kinase Akt, also known as protein kinase B. We show that mice deficient in Akt2 are impaired in the ability of insulin to lower blood glucose because of defects in the action of the hormone on liver and skeletal muscle. These data establish Akt2 as an essential gene in the maintenance of normal glucose homeostasis.

Prostaglandin F(2)alpha enhances glucose consumption through neither adipocyte differentiation nor GLUT1 expression in 3T3-L1 cells

Arachidonic acid (AA) at 0.2 mM enhances glucose uptake through increased levels of glucose transporter (GLUT) 1 protein in 3T3-L1 adipocytes. Since AA is a precursor of prostaglandins (PGs), we investigated the effect of PGs on glucose consumption in 3T3-L1 cells. Among several PGs, only prostaglandin F(2)alpha (PGF(2)alpha) enhanced glucose consumption in 3T3-L1 cells treated with dexamethasone (DEX), 3-isobutyl-1-methyl-xanthine (IBMX), and insulin. To study the mechanism of PGF(2)alpha-enhanced glucose consumption, we investigated the effect of PGF(2)alpha on glycerol-3-phosphate dehydrogenase (GPDH) activity, triglycerides (TGs) content, and the expression of GLUT1 protein. PGF(2)alpha suppressed GPDH activity and did not increase the expression of GLUT1 protein in 3T3-L1 cells treated with DEX, IBMX, and insulin. These results suggest that AA-stimulated glucose uptake is not through the effect of PGF(2)alpha. Our results indicate that PGF(2)alpha is a unique regulator of adipocyte differentiation (suppression) and glucose consumption (enhancement) in 3T3-L1 cells.

Immunoelectron microscopic evidence that GLUT4 translocation explains the stimulation of glucose transport in isolated rat white adipose cells

We used an improved cryosectioning technique in combination with quantitative immunoelectron microscopy to study GLUT4 compartments in isolated rat white adipose cells. We provide clear evidence that in unstimulated cells most of the GLUT4 localizes intracellularly to tubulovesicular structures clustered near small stacks of Golgi and endosomes, or scattered throughout the cytoplasm. This localization is entirely consistent with that originally described in brown adipose tissue, strongly suggesting that the GLUT4 compartments in white and brown adipose cells are morphologically similar. Furthermore, insulin induces parallel increases (with similar magnitudes) in glucose transport activity, approximately 16-fold, and cell-surface GLUT4, approximately 12-fold. Concomitantly, insulin decreases GLUT4 equally from all intracellular locations, in agreement with the concept that the entire cellular GLUT4 pool contributes to insulin-stimulated exocytosis. In the insulin-stimulated state, GLUT4 molecules are not randomly distributed on the plasma membrane, but neither are they enriched in caveolae. Importantly, the total number of GLUT4 C-terminal epitopes detected by the immuno-gold method is not significantly different between basal and insulin-stimulated cells, thus arguing directly against a reported insulin-induced unmasking effect. These results provide strong morphological evidence (1) that GLUT4 compartments are similar in all insulin-sensitive cells and (2) for the concept that GLUT4 translocation almost fully accounts for the increase in glucose transport in response to insulin.

Cellugyrin is a marker for a distinct population of intracellular Glut4-containing vesicles

Although Glut4 traffic is routinely described as translocation from an "intracellular storage pool" to the plasma membrane, it has been long realized that Glut4 travels through at least two functionally distinct intracellular membrane compartments on the way to and from the cell surface. Biochemical separation and systematic studies of the individual Glut4-containing compartments have been limited by the lack of appropriate reagents. We have prepared a monoclonal antibody against a novel component protein of Glut4 vesicles and have identified this protein as cellugyrin, a ubiquitously expressed homologue of a major synaptic vesicle protein, synaptogyrin. By means of sucrose gradient centrifugation, immunoadsorption, and confocal microscopy, we have shown that virtually all cellugyrin is co-localized with Glut4 in the same vesicles. However, unlike Glut4, cellugyrin is not re-distributed to the plasma membrane in response to insulin stimulation, and at least 40-50% of the total population of Glut4 vesicles do not contain this protein. We suggest that cellugyrin represents a specific marker of a functionally distinct population of Glut4 vesicles that permanently maintains its intracellular localization and is not recruited to the plasma membrane by insulin.

Insulin-responsive aminopeptidase trafficking in 3T3-L1 adipocytes

The insulin-responsive aminopeptidase (IRAP/VP165/gp160) was identified originally in GLUT4-containing vesicles and shown to translocate in response to insulin, much like the glucose transporter 4 (GLUT4). This study characterizes the trafficking and kinetics of IRAP in exocytosis, endocytosis, and recycling to the membrane in 3T3-L1 adipocytes. After exposure of 3T3-L1 adipocytes to insulin, IRAP translocated to the plasma membrane as assessed by either cell fractionation, surface biotinylation, or the plasma membrane sheet assay. The rate of exocytosis closely paralleled that of GLUT4. In the continuous presence of insulin, IRAP was endocytosed with a half-time of about 3-5 min. IRAP endocytosis is inhibited by cytosol acidification, a property of clathrin-mediated endocytosis, but not by the expression of a constitutively active Akt/PKB. Arrival in an LDM fraction derived via subcellular fractionation exhibited a slower time course than disappearance from the cell surface, suggesting additional endocytic intermediates. As assayed by membrane "sheets," GLUT4 and IRAP showed similar internalization rates that are wortmannin-insensitive and occur with a half-time of roughly 5 min. IRAP remaining on the cell surface 10 min following insulin removal was both biotin- and avidin-accessible, implying the absence of thin-necked invaginations. Finally, endocytosed IRAP quickly recycled back to the plasma membrane in a wortmannin-sensitive process. These results demonstrate rapid endocytosis and recycling of IRAP in the presence of insulin and trafficking that matches GLUT4 in rate.

Signaling pathways mediating insulin-stimulated glucose transport

A major action of insulin is to accelerate the rate of uptake of sugar into muscle and adipose cells following a meal. The biochemical mechanism by which this is accomplished has been a subject of intense experimentation, although elucidation of the pathways has remained elusive. In recent years, numerous signaling molecules and cascades modulated by insulin have been identified, although few have been definitively established as important to the metabolic actions of the hormone. An exception to this is the lipid kinase phosphatidylinositide 3'-kinase, which, under many conditions, appears absolutely required for insulin to stimulate hexose uptake into adipocytes. Akt/PKB, a serine/threonine protein kinase activated by insulin in a phosphatidylinositide 3'-kinase-dependent manner, has been implicated as a critical mediator of insulin's actions on metabolism and cell survival. Nonetheless, Akt/PKB's role in many insulin effects, particularly accelerated glucose transport, remains controversial. Interestingly, soluble analogues of ceramide antagonize both insulin's activation of Akt/PKB as well as its stimulation of glucose transport, consistent with a causal relationship between the two.

Insulin regulation of protein traffic in rat adipose cells

Rat adipocytes were biotinylated with cell-impermeable reagents, sulfo-N-hydroxysuccinimide-biotin and sulfo-N-hydroxysuccinimide-S-S-biotin in the absence and presence of insulin. Biotinylated and nonbiotinylated populations of the insulin-like growth factor-II/mannose 6-phosphate receptor, the transferrin receptor, and insulin-responsive aminopeptidase were separated by adsorption to streptavidin-agarose to determine the percentage of the biotinylated protein molecules versus their total amount in different subcellular compartments. Results indicate that adipose cells possess at least two distinct cell surface recycling pathways for insulin-like growth factor-II/mannose 6-phosphate receptor (MPR) and transferrin receptor (TfR): one which is mediated by glucose transporter isoform 4(Glut4)-vesicles and another that bypasses this compartment. Under basal conditions, the first pathway is not active, and cell surface recycling of TfR and, to a lesser extent, MPR proceeds via the second pathway. Insulin dramatically stimulates recycling through the first pathway and has little effect on the second. Within the Glut4-containing compartment, insulin has profoundly different effects on intracellular trafficking of insulin-responsive aminopeptidase on one hand and MPR and TfR on the other. After insulin administration, insulin-responsive aminopeptidase is redistributed from Glut4-containing vesicles to the plasma membrane and stays there for at least 30 min with minimal detectable internalization and recycling, whereas MPR and TfR rapidly shuttle between Glut4 vesicles and the plasma membrane in such a way that after 30 min of insulin treatment, virtually every receptor molecule in this compartment completes at least one trafficking cycle to the cell surface. Thus, different recycling proteins, which compose Glut4-containing vesicles, are internalized into this compartment at their own distinctive rates.

Membrane trafficking of neurotransmitter transporters in the regulation of synaptic transmission

Many psychoactive drugs influence the transport of neurotransmitters across biological membranes, suggesting that the physiological regulation of neurotransmitter transport might contribute to normal and perhaps abnormal behaviour. Over the past few years, molecular characterization of the neurotransmitter transporters has enabled investigation of their subcellular location and regulation. The analysis of location suggests that membrane trafficking has an important role in the normal function of these proteins. One of the major regulatory mechanisms also involves changes in localization that might contribute to synaptic plasticity. This article discusses recent work on the membrane trafficking of neurotransmitter transporters and its role in regulating their activity.

Identification of wortmannin-sensitive targets in 3T3-L1 adipocytes. DissociationoOf insulin-stimulated glucose uptake and glut4 translocation

The current studies investigated the contribution of phosphatidylinositol 3-kinase (PI3-kinase) isoforms to insulin-stimulated glucose uptake and glucose transporter 4 (GLUT4) translocation. Experiments involving the microinjection of antibodies specific for the p110 catalytic subunit of class I PI3-kinases demonstrated an absolute requirement for this form of the enzyme in GLUT4 translocation. This finding was confirmed by the demonstration that the PI3-kinase antagonist wortmannin inhibits GLUT4 and insulin-responsive aminopeptidase translocation with a dose response identical to that required to inhibit another class I PI3-kinase-dependent event, activation of pp70 S6-kinase. Interestingly, wortmannin inhibited insulin-stimulated glucose uptake at much lower doses, suggesting the existence of a second, higher affinity target of the drug. Subsequent removal of wortmannin from the media shifted this dose-response curve to one resembling that for GLUT4 translocation and pp70 S6-kinase. This is consistent with the lower affinity target being p110, which is irreversibly inhibited by wortmannin. Wortmannin did not reduce glucose uptake in cells stably expressing Myr-Akt, which constitutively induced GLUT4 translocation to the plasma membrane; this demonstrates that wortmannin does not inhibit the transporters directly. In addition to elucidating a second wortmannin-sensitive pathway in 3T3-L1 adipocytes, these studies suggest that the presence of GLUT4 on the plasma membrane is not sufficient for activation of glucose uptake.

Differentiation-dependent suppression of platelet-derived growth factor signaling in cultured adipocytes

A critical component of vertebrate cellular differentiation is the acquisition of sensitivity to a restricted subset of peptide hormones and growth factors. This accounts for the unique capability of insulin (and possibly insulin-like growth factor-1), but not other growth factors, to stimulate glucose uptake and anabolic metabolism in heart, skeletal muscle, and adipose tissue. This selectivity is faithfully recapitulated in the cultured adipocyte line, 3T3-L1, which responds to insulin, but not platelet-derived growth factor (PDGF), with increased hexose uptake. The serine/threonine protein kinases Akt1 and Akt2, which have been implicated as mediators of insulin-stimulated glucose uptake, as well as glycogen, lipid, and protein synthesis, were shown to mirror this selectivity in this tissue culture system. This was particularly apparent in 3T3-L1 adipocytes overexpressing an epitope-tagged form of Akt2 in which insulin activated Akt2 10-fold better than PDGF. Similarly, in 3T3-L1 adipocytes, only insulin stimulated phosphorylation of Akt's endogenous substrate, GSK-3beta. Other signaling molecules, including phosphatidylinositol 3-kinase, pp70 S6-kinase, mitogen-activated protein kinase, and PHAS-1/4EBP-1, did not demonstrate this selective responsiveness to insulin but were instead activated comparably by both insulin and PDGF. Moreover, concurrent treatment with PDGF and insulin did not diminish activation of phosphatidylinositol 3-kinase, Akt, or glucose transport, indicating that PDGF did not simultaneously activate an inhibitory mechanism. Interestingly, PDGF and insulin comparably stimulated both Akt isoforms, as well as numerous other signaling molecules, in undifferentiated 3T3-L1 preadipocytes. Collectively, these data suggest that differential activation of Akt in adipocytes may contribute to insulin's exclusive mediation of the metabolic events involved in glucose metabolism. Moreover, they suggest a novel mechanism by which differentiation-dependent hormone selectivity is conferred through the suppression of specific signaling pathways operational in undifferentiated cell types.

Regulation of glucose transport by hypoxia

Transport of glucose into most mammalian cells and tissues is rate-controlling for its metabolism. Glucose transport is acutely stimulated by hypoxic conditions, and the response is mediated by enhanced function of the facilitative glucose transporters (Glut), Glut1, Glut3, and Glut4. The expression and activity of the Glut-mediated transport is coupled to the energetic status of the cell, such that the inhibition of oxidative phosphorylation resulting from exposure to hypoxia leads to a stimulation of glucose transport. The premise that the glucose transport response to hypoxia is secondary to inhibition of mitochondrial function is supported by the finding that exposure of a variety of cells and tissues to agents such as azide or cyanide, in the presence of oxygen, also leads to stimulation of glucose transport. The mechanisms underlying the acute stimulation of transport include translocation of Gluts to the plasma membrane (Glut1 and Glut4) and activation of transporters pre-exiting in the plasma membrane (Glut1). A more prolonged exposure to hypoxia results in enhanced transcription of the Glut1 glucose transporter gene, with little or no effect on transcription of other Glut genes. The transcriptional effect of hypoxia is mediated by dual mechanisms operating in parallel, namely, (1) enhancement of Glut1 gene transcription in response to a reduction in oxygen concentration per se, acting through the hypoxia-signaling pathway, and (2) stimulation of Glut1 transcription secondary to the associated inhibition of oxidative phosphorylation during hypoxia. Among the various hypoxia-responsive genes, Glut1 is the first gene whose rate of transcription has been shown to be dually regulated by hypoxia. In addition, inhibition of oxidative phosphorylation per se, and not the reduction in oxygen tension itself, results in a stabilization of Glut1 mRNA. The increase in cell Glut1 mRNA content, resulting from its enhanced transcription and decreased degradation, leads to increased cell and plasma membrane Glut1 content, which is manifested by a further stimulation of glucose transport during the adaptive response to prolonged exposure to hypoxia.

The role of glycogen synthase kinase 3beta in insulin-stimulated glucose metabolism

To characterize the contribution of glycogen synthase kinase 3beta (GSK3beta) inactivation to insulin-stimulated glucose metabolism, wild-type (WT-GSK), catalytically inactive (KM-GSK), and uninhibitable (S9A-GSK) forms of GSK3beta were expressed in insulin-responsive 3T3-L1 adipocytes using adenovirus technology. WT-GSK, but not KM-GSK, reduced basal and insulin-stimulated glycogen synthase activity without affecting the -fold stimulation of the enzyme by insulin. S9A-GSK similarly decreased cellular glycogen synthase activity, but also partially blocked insulin stimulation of the enzyme. S9A-GSK expression also markedly inhibited insulin stimulation of IRS-1-associated phosphatidylinositol 3-kinase activity, but only weakly inhibited insulin-stimulated Akt/PKB phosphorylation and glucose uptake, with no effect on GLUT4 translocation. To further evaluate the role of GSK3beta in insulin signaling, the GSK3beta inhibitor lithium was used to mimic the consequences of insulin-stimulated GSK3beta inactivation. Although lithium stimulated the incorporation of glucose into glycogen and glycogen synthase enzyme activity, the inhibitor was without effect on GLUT4 translocation and pp70 S6 kinase. Lithium stimulation of glycogen synthesis was insensitive to wortmannin, which is consistent with its acting directly on GSK3beta downstream of phosphatidylinositol 3-kinase. These data support the hypothesis that GSK3beta contributes to insulin regulation of glycogen synthesis, but is not responsible for the increase in glucose transport.

Cloning and characterization of a 22 kDa protein from rat adipocytes: a new member of the reticulon family

In the course of our examination of proteins associated with the GLUT4-containing vesicles of rat adipocytes we have identified a new 22 kDa member of the family of endoplasmic reticulum (ER) proteins known as reticulons. The protein, which we refer to as vp20, was purified from a preparation of GLUT4-containing vesicles of rat adipocytes, and tryptic peptides were micro-sequenced. From this information a cDNA encoding a single open reading frame for a protein of 22 kDa was cloned. This protein is homologous to known members of the reticulon protein family. vp20 has two hydrophobic stretches of about 35 amino acids that could be membrane spanning domains and an ER retention motif at its carboxy-terminus. vp20 was most abundant in the high density microsome fraction of adipocytes, which is the fraction most enriched in ER. Only a small fraction of vp20 was present in the GLUT4 vesicle population, and that fraction appears to be due to ER vesicles that were non-specifically bound to the adsorbent. Analysis of tissue distribution of vp20 in rats revealed that it is concentrated in muscle, fat and the brain.

Novel aspects of the regulation of glycogen storage

The storage polysaccharide glycogen is widely distributed in nature, from bacteria to mammals. Study of its regulated accumulation has resulted in the discovery or elaboration of several important biochemical principles. Many aspects of the control of glycogen storage still remain poorly understood and glycogen metabolism continues to provide interesting models of more general relevance.

Akt-2 binds to Glut4-containing vesicles and phosphorylates their component proteins in response to insulin

Glut4-containing vesicles immunoadsorbed from primary rat adipocytes possess endogenous protein kinase activity and phosphorylation substrates. Phosphorylation of several vesicle proteins including Glut4 itself is rapidly activated by insulin. Wortmannin blocks the effect of insulin when added to cells in vivo prior to insulin administration. By means of MonoQ chromatography and Western blot analysis, vesicle-associated protein kinase is identified as Akt-2, a lipid-binding protein kinase involved in insulin signaling. Akt-2 is found to be recruited to Glut4-containing vesicles in response to insulin.

Synergistic effect of arachidonic acid and cyclic AMP on glucose transport in 3T3-L1 adipocytes

The combined effect of arachidonic acid and cAMP on glucose transport was examined in 3T3-L1 adipocytes. In cells pre-treated with arachidonic acid and increasing concentrations of 8-bromo cAMP for 8 h, although either agent alone enhanced glucose uptake, the simultaneous presence of both agents dramatically increased 2-deoxyglucose uptake in a synergistic fashion. Insulin-stimulated glucose transport, on the other hand, was only slightly affected. The synergistic effect of these two agents was abolished in the presence of cycloheximide. Immunoblot analysis revealed that the contents of ubiquitous glucose transporter (GLUT1) in total cellular and plasma membranes were similarly augmented in cells pre-treated with both arachidonic acid and 8-bromo cAMP, to a greater extent than the additive effect of each agent alone. The content of GLUT4, on the other hand, was not altered under the same experimental conditions. In cells pre-treated with 4beta-phorbol 12beta-myristate 13alpha-acetate (PMA) for 24 h to down-regulate protein kinase C (PKC), the subsequent synergistic effect of arachidonic acid and 8-bromo cAMP was greatly inhibited. In addition, pre-treatment with both PMA and 8-bromo cAMP enhanced glucose transport in a similarly synergistic fashion. Thus the present study seems to indicate that arachidonic acid may act with cAMP in a synergistic way to increase glucose transport by a PKC-dependent mechanism. The increased activity may be accounted for by increased GLUT1 synthesis.

Early biochemical events in insulin-stimulated fluid phase endocytosis

We examined the initial molecular mechanisms by which cells nonselectively internalize extracellular solutes in response to insulin. Insulin-stimulated fluid phase endocytosis (FPE) was examined in responsive cells, and the roles of the insulin receptor, insulin receptor substrate-1 (IRS-1), phosphatidylinositol 3'-kinase (PI 3'-kinase), Ras, and mitogen-activated protein kinase kinase (MEK) were assessed. Active insulin receptors were essential, as demonstrated by the stimulation of FPE by insulin in HIRc-B cells (Rat-1 cells expressing 1.2 x 10(6) normal insulin receptors/cell) but not in untransfected Rat-1 cells or in Rat-1 cells expressing the inactive A/K1018 receptor. IRS-1 expression augmented insulin-stimulated FPE, as assessed in 32D cells, a hematopoietic precursor cell line lacking endogenous IRS-1. Insulin-stimulated FPE was inhibited in mouse brown adipose tissue (BAT) cells expressing the 17N dominant negative mutant Ras and was augmented in cells expressing wild-type Ras. The MEK inhibitor PD-98059 had little effect on insulin-stimulated FPE in BAT cells. In 32D cells, but not in HIRc-B and BAT cells, insulin-stimulated FPE was inhibited by 10 nM wortmannin, an inhibitor of PI 3'-kinase. The results indicate that the insulin receptor, IRS-1, Ras, and, perhaps in certain cell types, PI 3'-kinase are involved in mediating insulin-stimulated FPE.

Snareing GLUT4 at the plasma membrane in muscle and fat

Explosive advances in the understanding of vesicle trafficking between intracellular compartments have occurred in recent years. These investigations inspired an attractive model for intracellular membrane transport, referred as the SNARE hypothesis. These advances have been profitably applied to one system in muscle and fat; the regulation of intracellular trafficking of the insulin-regulatable facilitative glucose transporter (GLUT4). Investigations in insulin-sensitive cell types revealed a remarkable conservation in the mechanism of vesicular transport between synaptic vesicles in the presynaptic nerve terminal and GLUT4-containing vesicles in muscle and fat. On the other hand, unique players in insulin-regulatable GLUT4 movement have also been clarified during this process. Thus, unveiling the molecular mechanisms regulating insulin-stimulated GLUT4 trafficking will significantly contribute to our understanding of whole body glucose homeostasis as well as the cell biology of protein trafficking, membrane dynamics, and organelle biogenesis.

Insulin-dependent protein trafficking in skeletal muscle cells

We have established a simple procedure for the separation of intracellular pool(s) of glucose transporter isoform GLUT-4-containing vesicles from the surface sarcolemma and T tubule membranes of rat skeletal myocytes. This procedure enabled us to immunopurify intracellular GLUT-4-containing vesicles and to demonstrate that 20-30% of the receptors for insulin-like growth factor II/mannose 6-phosphate and transferrin are colocalized with GLUT-4 in the same vesicles. Using our new fractionation procedure as well as cell surface biotinylation, we have shown that these receptors are translocated from their intracellular compartment(s) to the cell surface along with GLUT-4 after insulin stimulation in vivo. Denervation causes a considerable downregulation of GLUT-4 protein in skeletal muscle but does not affect the level of expression of other known component proteins of the corresponding vesicles. Moreover, the sedimentation coefficient of these vesicles remains unchanged by denervation. We suggest that the normal level of GLUT-4 expression is not necessary for the structural organization and insulin-sensitive translocation of its cognate intracellular compartment.

Exercise, glucose transport, and insulin sensitivity

Physical exercise can be an important adjunct in the treatment of both non-insulin-dependent diabetes mellitus and insulin-dependent diabetes mellitus. Over the past several years, considerable progress has been made in understanding the molecular basis for these clinically important effects of physical exercise. Similarly to insulin, a single bout of exercise increases the rate of glucose uptake into the contracting skeletal muscles, a process that is regulated by the translocation of GLUT4 glucose transporters to the plasma membrane and transverse tubules. Exercise and insulin utilize different signaling pathways, both of which lead to the activation of glucose transport, which perhaps explains why humans with insulin resistance can increase muscle glucose transport in response to an acute bout of exercise. Exercise training in humans results in numerous beneficial adaptations in skeletal muscles, including an increase in GLUT4 expression. The increase in muscle GLUT4 in trained individuals contributes to an increase in the responsiveness of muscle glucose uptake to insulin, although not all studies show that exercise training in patients with diabetes improves overall glucose control. However, there is now extensive epidemiological evidence demonstrating that long-term regular physical exercise can significantly reduce the risk of developing non-insulin-dependent diabetes mellitus.

4-Bromocrotonic acid enhances basal but inhibits insulin-stimulated glucose transport in 3T3-L1 adipocytes

Inhibitors of fatty acid oxidation, 2-bromopalmitic acid (Br-C16) and 4-bromocrotonic acid (Br-C4) were examined for their effect on glucose transport in 3T3-L1 adipocytes. Whereas Br-C16 was without effect, Br-C4 augmented basal but inhibited insulin-stimulated 2-deoxyglucose uptake in a dose- and time-dependent manner. Immunoblot analysis indicated that following Br-C4 pretreatment, the content of GLUT1 in plasma membranes was increased whereas insulin-induced translocation of GLUT4 was greatly eliminated. The total cellular amount of GLUT1 or GLUT4, on the other hand, was not altered. Thus these results seem to suggest that Br-C4 has opposite effect on basal and insulin-stimulated glucose transport by a mechanism other than its inhibition of fatty acid oxidation. The translocation processes for both GLUT1 and GLUT4 transporters appears to be altered.

Sortilin is the major 110-kDa protein in GLUT4 vesicles from adipocytes

Vesicles containing the glucose transporter GLUT4 from rat adipocytes contain a major protein of 110 kDa. We have isolated this protein, obtained the sequences of peptides, and cloned a large portion of its cDNA. This revealed that the protein is sortilin, a novel membrane protein that was cloned in another context from a human source while this work was in progress. Subcellular fractionation of rat and 3T3-L1 adipocytes, together with GLUT4 vesicle isolation, showed that sortilin was primarily located in the low density microsomes in vesicles containing GLUT4. Insulin caused a 1.7-fold increase in the amount of sortilin at the plasma membranes of 3T3-L1 adipocytes, as assessed by cell surface biotinylation. The expression of sortilin in 3T3-L1 cells occurred only upon differentiation. Previous characterization of sortilin has led to the suggestion that it functions to sort lumenal proteins from the trans Golgi. The significance of its insulin-stimulated increase at the cell surface and of its expression upon differentiation will require definitive delineation of its function.

Phosphoinositide 3-kinase is required for insulin-induced but not for growth hormone- or hyperosmolarity-induced glucose uptake in 3T3-L1 adipocytes

The(1) regulatory mechanism of glucose uptake in 3T3-L1 adipocytes was investigated with the use of recombinant adenovirus vectors encoding various dominant negative proteins. Infection with a virus encoding a mutant regulatory subunit of phosphoinositide (PI) 3-kinase that does not bind the 110-kDa catalytic subunit (delta p85) inhibited the insulin-induced increase in PI 3-kinase activity co-precipitated by antibodies to phosphotyrosine and glucose uptake in a virus dose-dependent manner. Overexpression of a dominant negative RAS mutant in which Asp57 is replaced with tyrosine (RAS57Y) or of a dominant negative SOS mutant that lacks guanine nucleotide exchange activity (delta SOS) abolished the insulin-induced increase in mitogen-activated protein kinase activity, but had no effect on PI 3-kinase activity or glucose uptake. Although GH and hyperosmolarity attributable to 300 mM sorbitol each promoted glucose uptake and translocation of glucose transporter (GLUT)4 to an extent comparable to that of insulin, these stimuli triggered little or no association of PI 3-kinase activity with tyrosine-phosphorylated proteins. Overexpression of delta p85 or treatment of cells with wortmannin, an inhibitor of PI 3-kinase activity, had no effect on glucose uptake or translocation of GLUT4 stimulated by GH or hyperosmolarity. Moreover, overexpression of delta SOS or RAC17N also did not affect the increase in glucose uptake induced by these stimuli. A serine/threonine kinase Akt, a constitutively active mutant of which was previously shown to stimulate glucose uptake, is activated by insulin, GH, and hyperosmolarity to approximately 4-fold, approximately 2.1-fold, and approximately 2.3-fold over basal level, respectively. These results suggest that insulin-induced but neither GH- or hyperosmolarity-induced glucose uptake is PI 3-kinase-dependent, and neither RAS nor RAC is required for glucose uptake induced by these stimuli in 3T3-L1 adipocytes.

Glucose transporter GLUT3 in the rat placental barrier: a possible machinery for the transplacental transfer of glucose

Glucose transfer across the placental barrier is crucial for fetal development. To investigate the role of glucose transporter isoforms in the transplacental transfer of glucose, we investigated the localization of glucose transporters GLUT1 and GLUT3 immunohistochemically in the rat placenta. In the labyrinth, the site of maternofetal exchange of substances, both GLUT1 and GLUT3 were present, whereas only GLUT1 was detected in the junctional region. In the labyrinthine wall, which lies between maternal and fetal circulations, GLUT3 exhibited polarized localization; i.e. it was present at the plasma membranes of the maternal blood side in the syncytiotrophoblast layers. GLUT1 was concentrated at plasma membranes of the maternal and fetal blood sides of syncytiotrophoblast layers. The asymmetric distribution of GLUT3 across the placental barrier may suggest asymmetric transfer of glucose, which would be beneficial to provide a stable milieu for fetal development.

A synthetic peptide corresponding to the GLUT4 C-terminal cytoplasmic domain causes insulin-like glucose transport stimulation and GLUT4 recruitment in rat adipocytes

In rat epididymal adipocytes, practically all of the major glucose transporter isoform GLUT4 is constitutively sequestered in intracellular membranes and moves to the plasma membrane in response to insulin, whereas about half of GLUT1, the minor isoform, is constitutively functional at the plasma membrane and thus less affected by insulin. Transfection studies using cells whose glucose transport is normally not regulated by insulin have suggested that the C-terminal cytoplasmic domain of GLUT4 is responsible for its constitutive intracellular sequestration. To test if this was also the case in a classical insulin target cell, we introduced synthetic peptides corresponding to the C-terminal cytoplasmic domain of GLUT4 and GLUT1 (GLUT4C and GLUT1C, respectively) into rat adipocytes and studied their effects on the glucose transport activity and the steady state GLUT4 and GLUT1 distribution between the plasma membrane and intracellular membranes in host cells. GLUT4C introduced into basal adipocytes caused a large (up to 4.5-fold) and dose-dependent increase in the plasma membrane GLUT4, with a proportional reduction in microsomal GLUT4, without affecting GLUT1 distribution. GLUT4C incorporation also caused a large (up to 3-fold) dose-dependent stimulation of 3-O-methyl D-glucose (3OMG) flux in host cells. GLUT4C caused little if any GLUT4 or GLUT1 redistribution and changes in 3OMG flux in insulin-stimulated adipocytes. GLUT1C, on the other hand, did not affect GLUT1 or GLUT4 targeting and 3OMG flux in host cells. These findings not only underscore the importance of the C-terminal cytoplasmic domain of GLUT4 in its constitutive intracellular sequestration in a classical insulin target cell but also suggest the existence of a regulatory protein in adipocytes that interacts with GLUT4 at its cytoplasmic domain, thus participating in the constitutive intracellular sequestration of GLUT4.

Immunocytochemical evidence that GLUT4 resides in a specialized translocation post-endosomal VAMP2-positive compartment in rat adipose cells in the absence of insulin

Insulin stimulates glucose transport in rat adipose cells through the translocation of GLUT4 from a poorly defined intracellular compartment to the cell surface. We employed confocal microscopy to determine the in situ localization of GLUT4 relative to vesicle, Golgi, and endosomal proteins in these physiological insulin target cells. Three-dimensional analyses of GLUT4 immunostaining in basal cells revealed an intracellular punctate, patchy distribution both in the perinuclear region and scattered throughout the cytoplasm. VAMP2 closely associates with GLUT4 in many punctate vesicle-like structures. A small fraction of GLUT4 overlaps with TGN38-mannosidase II, gamma-adaptin, and mannose-6-phosphate receptors in the perinuclear region, presumably corresponding to late endosome and trans-Golgi network structures. GLUT4 does not co-localize with transferrin receptors, clathrin, and Igp-120. After insulin treatment, GLUT4 partially redistributes to the cell surface and decreases in the perinuclear area. However, GLUT4 remains co-localized with TGN38-mannosidase II and gamma-adaptin. Therefore, the basal compartment from which GLUT4 is translocated in response to insulin comprises specialized post-endosomal VAMP2-positive vesicles, distinct from the constitutively recycling endosomes. These results are consistent with a kinetic model in which GLUT4 is sequestered through two or more intracellular pools in series.

The molecular genetics of hexose transport in yeasts

Transport across the plasma membrane is the first, obligatory step of hexose utilization. In yeast cells the uptake of hexoses is mediated by a large family of related transporter proteins. In baker's yeast Saccharomyces cerevisiae the genes of 20 different hexose transporter-related proteins have been identified. Six of these transmembrane proteins mediate the metabolically relevant uptake of glucose, fructose and mannose for growth, two others catalyze the transport of only small amounts of these sugars, one protein is a galactose transporter but also able to transport glucose, two transporters act as glucose sensors, two others are involved in the pleiotropic drug resistance process, and the functions of the remaining hexose transporter-related proteins are not yet known. The catabolic hexose transporters exhibit different affinities for their substrates, and expression of their corresponding genes is controlled by the glucose sensors according to the availability of carbon sources. In contrast, milk yeast Kluyveromyces lactis contains only a few different hexose transporters. Genes of other monosaccharide transporter-related proteins have been found in fission yeast Schizosaccharomyces pombe and in the xylose-fermenting yeast Pichia stipitis. However, the molecular genetics of hexose transport in many other yeasts remains to be established. The further characterization of this multigene family of hexose transporters should help to elucidate the role of transport in yeast sugar metabolism.

Mechanism of glucose transport across the human and rat placental barrier: a review

Glucose is one of the most important substances transferred from the maternal blood to the fetal circulation in the placenta, and its transport across the cellular membranes is mediated by glucose transporters. Facilitated-diffusion glucose transporter GLUT1 is abundant in the placental barrier, as is the case in other blood-tissue barriers, where GLUT1 is present at the critical plasma membranes of the barrier cells. In the human placenta, the microvillous apical and the basal plasma membranes of the syncytiotrophoblast are rich in GLUT1, which molecule seems to be responsible for the transcellular transport of glucose across the placental barrier. In the rat placental labyrinth, two layers of syncytiotrophoblasts (termed syncytiotrophoblasts I and II from the maternal side) serve as a barrier. GLUT1 is abundant at the plasma membrane of syncytiotrophoblast I facing the maternal side, and the plasma membrane of syncytiotrophoblast II facing the fetal side. Numerous gap junctions, made of connexin 26, connect syncytiotrophoblasts I and II, comprising a channel for the transfer of glucose between them. GLUT1 in combination with the gap junction, therefore, seems to serve as the structural basis for the transport of glucose across the rat placental barrier.

Roles of PI 3-kinase and Ras on insulin-stimulated glucose transport in 3T3-L1 adipocytes

The dominant negative p85alpha regulatory subunit (delta p85alpha) of phosphatidylinositol (PI) 3-kinase or dominant negative Ras (N17Ras) was overexpressed in 3T3-L1 adipocytes using an adenovirus-mediated gene transduction system. Functional expression of delta p85alpha and N17Ras was confirmed by marked inhibition of insulin-stimulated PI 3-kinase activity and mitogen-activated protein kinase activity, respectively. N17Ras expression did not affect glucose transport activity, whereas delta p85alpha expression inhibited insulin-stimulated glucose transport with impairment of GLUT-4 translocation, although inhibition of glucose transport activity was less remarkable than that of PI 3-kinase activity in delta p85alpha-expressing cells. Thus the Ras signaling pathway does not play a major part in either translocation or intrinsic activity of glucose transporters, but PI 3-kinase activation, via phosphotyrosyl proteins and heterodimeric PI 3-kinase, plays a pivotal role in insulin-stimulated glucose transport. However, a discrepancy was observed between PI 3-kinase activity and glucose transport activity, suggesting a possibility that a different pathway(s) is involved in insulin-stimulated intrinsic activity of glucose transporters.

Transport of glucose across the blood-tissue barriers

In specialized parts of the body, free exchange of substances between blood and tissue cells is hindered by the presence of a barrier cell layer(s). Specialized milieu of the compartments provided by these "blood-tissue barriers" seems to be important for specific functions of the tissue cells guarded by the barriers. In blood-tissue barriers, such as the blood-brain barrier, blood-cerebrospinal fluid barrier, blood-nerve barrier, blood-retinal barrier, blood-aqueous barrier, blood-perilymph barrier, and placental barrier, endothelial or epithelial cells sealed by tight junctions, or a syncytial cell layer(s), serve as a structural basis of the barrier. A selective transport system localized in the cells of the barrier provides substances needed by the cells inside the barrier. GLUT1, an isoform of facilitated-diffusion glucose transporters, is abundant in cells of the barrier. GLUT1 is concentrated at the critical plasma membranes of cells of the barriers and thereby constitutes the major machinery for the transport of glucose across these barriers where transport occurs by a transcellular mechanism. In the barrier composed of double-epithelial layers, such as the epithelium of the ciliary body in the case of the blood-aqueous barrier, gap junctions appear to play an important role in addition to GLUT1 for the transfer of glucose across the barrier.

Wortmannin converts insulin but not oxytocin from an antilipolytic to a lipolytic agent in the presence of forskolin

Insulin is an important regulator of glucose transport and lipolysis in adipocytes. The present studies compared the effects of insulin in rat adipocytes with the effects of oxytocin and peroxovanadate, which mimic some effects of insulin. The antilipolytic effects of peroxovanadate and oxytocin were unaffected by 500 nmol/L wortmannin, which blocked the antilipolytic action of insulin. However, wortmannin, which is a relatively specific inhibitor of phosphatidylinositol 3-kinase, did block most of the stimulation of glucose metabolism by peroxovanadate while having little effect on that due to oxytocin. Under appropriate conditions, it was also possible to demonstrate a lipolytic action of insulin, especially with low (0.1 to 1 nmol/L) concentrations of insulin after exposure of adipocytes to 50 nmol/L wortmannin. The data provide additional support for the hypothesis that oxytocin and peroxovanadate affect adipose tissue metabolism by mechanisms distinctly different from those involved in insulin action.